Project: Installing a late model electronic HVAC control unit in a 1985 El Camino.

Problem: The AC request that comes from the HVAC head is 12V, but very low current ( I believe- I'm not Elecronics savy). When the HVAC requires that the AC compressor turns on, this line goes from 0V to Ign Voltage ( approx 12.8v). The diagram in the Service manual shows this line as IGN ---1000K ---out to controller. I think that going through the 1K resistor is protection, and also lowers the current available.

Initially, not knowing that his was low current, I tried hooking it up to an automotive relay, which would then drive the AC compressor. This relay has a current of approx 150 MA. Did not work.

Then I tried a PCB mounted relay, drawing 30 MA. Did not work
Next I tried a PCB mounted relay drawing 15 MA. Did not work.

Somehow, I need a solution that will take the 12v+, very low current signal coming from the HVAC and convert that signal to a means to drive a 12V 150MA relay. Theoretically, either of the PCB relays I have should be able to drive the Auto relay. Even the MOSFET is supposed to have enough capacity ( 600MA) to be able to drive the coil on the relay directly. I guess I'm just having problems figuring out how to take the signal from the HVAC unit and use that to drive something else.

Note: All wiring is being done inside the vehicle. All through-firewall wiring uses existing wiring from the manual HVAC system.

The MOSFET should have worked nicely. In fact the right MOSFET might allow you to eliminate the relay altogether. Can you provide a picture of what you did, or maybe a schematic (your's not the datasheet)? How certain are you that the pins were not reversed? (It's easy to do!)

You might also look up the limits on the gate voltage for that MOSFET. It's meant for logic level (5v) and I doubt that 15v would damage it, but maybe worth a look. MOSFETs are also notorious for being vulnerable to static damage.

Wayneh, thanks for the quick response. Here's a dwg of what I have constructed. I will go back and check my wiring one more time. Do you think that the low current of the output from the HVAC controller is sufficient to drive these devices?

Just for grins I went out and grounded the PCB relay coil, and it closed the circuit to the Auto AC Relay. Therefore, the problem is with the MOSFET. I measured the voltage of the input going into the MOSFET. With the MOSFET disconnected from the circuit, I get 12.86 volts on the input line. When I connect the MOSFET, I measure .55 volts. I am confused. Guess that happens when a mechanical guy tries to go electrical!

Just for grins I went out and grounded the PCB relay coil, and it closed the circuit to the Auto AC Relay. Therefore, the problem is with the MOSFET. I measured the voltage of the input going into the MOSFET. With the MOSFET disconnected from the circuit, I get 12.86 volts on the input line. When I connect the MOSFET, I measure .55 volts. I am confused. Guess that happens when a mechanical guy tries to go electrical!

Click to expand...

My read of the datasheet shows the gate is the center pin. You've drawn it as the source, which should be grounded. If you've hooked it up that way, bingo.

With the flat face towards you and the pins down, the MOSFET pins are Source-Gate-Drain. In your drawing, the pins from left to right should be Drain (the bottom, low voltage side of the relay), Source (ground), Gate (signal).

I don't think you'll be able to replace the relay with that MOSFET, as the magnetic clutch on the A/C compressor probably draws a good bit of current - not sure how much offhand, but 5A or more wouldn't surprise me. The little MOSFET would be toasted.

I was referring to eliminating the PCB relay being used to trip the larger, "normal" relay that controls the AC. The latter, larger relay could probably be replaced by a MOSFET too, but it would indeed need to be much bigger than the little one cited here, and there's really no good reason to modify the existing system. As I understand it, the OP just needed to use a logic signal to trip an existing relay instead of whatever signal the car was designed for. Solution is one little MOSFET, wired properly.